code: def test_external_registry():
      def getter(key: str):
          return key
  
      registry = ExternalRegistry(getter, "backbones", "test_provider")
      assert registry.get("testing")() == "testing"
      available = registry.available_keys()
      assert len(available) == 1
      assert "test_provider" in available[0]
  
      registry = ExternalRegistry(
          getter, "backbones", ["test_provider_1", "test_provider_2"]
      )
      assert (
          "test_provider_1, test_provider_2"
          in registry.available_keys()[0]
      )
  
      registry = ExternalRegistry(getter, "backbones")
      assert len(registry.available_keys()) == 0
  
file_path: lightning-flash/tests/core/test_registry.py
imports: import logging
  import pytest
  from flash.core.registry import ConcatRegistry, ExternalRegistry, FlashRegistry
  from flash.core.utilities.imports import _TOPIC_CORE_AVAILABLE
  from torch import nn
label_desc: Use image node_id 4 for calling a global function with example usage: test_external_registry() and returns: key